build(deps): bump sphinxcontrib-programoutput from 0.17 to 0.18 in /lib/spack/docs #775

Job	Run time
changes	4s
prechecks / bootstrap-dev-rhel8	4m 30s
prechecks / validate	36s
prechecks / style	54s
prechecks / import-check	52s
prechecks / audit / package-audits (windows-latest, powershell Invoke-Expression -Command "./share/spack/qa/windows_t...	5m 11s
prechecks / audit / package-audits (ubuntu-latest, bash)	8m 41s
prechecks / audit / package-audits (macos-latest, bash)	5m 31s
all-prechecks	1s
unit-tests	0s
bootstrap	0s
coverage	0s
all	1s
	26m 21s

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